1.0 Field of the Invention
The present invention relates to frequency detection and, more particularly, to a circuit for narrow band frequency detection particularly suited for measuring, even in the presence of interfering frequencies and noise, the frequencies components contained in a tone burst.
2.0 Description of the Related Art
Presently information contained in toned burst transmitted to devices, such as sonobuoys, are detected by tone decoders comprised of integrated circuits which, in their classical form are made up of analog components. The tone burst is commonly used as a transmitted signal consisting of a two tone sinewave sustained for a short duration from about 50 to 500 milliseconds, representing the sonobuoy address, followed by a single tone for a duration from about 50 to 1000 millisecond representing the sonobuoy command. Whereas the tone decoders provide an indication of the existence of the tone burst, even in the presence of interfering frequencies and noise, the analog components of the integrated circuit can drift and age. The utilization of the tone decoder having analog components typically requires the manual setting of a free-running frequency of the tone decoder and, therefore, is individually calibrated. The utilization of the tone decoder detecting the information, such as the address and command determination of the associated sonobuoy, commonly requires a separate tone detector circuit for each command identification contained in the tone burst to be decoded. It is desired that a tone decoder be provided that does not suffer the drawbacks created by drifting and aging analog components, does not require the need for individual calibration, and does not require a separate tone detector to identify the individual frequencies components, (e.g., address and command) contained in the tone burst. Further, it is desired that the tone decoder provide additional capabilities. For example, by having the tone detector measure component frequency in the one burst, the capture range, known in the art, is effectively narrowed as compared to a conventional tone decoder.
Another approach used in the detection of information, such as sonobuoy address information contained in the tone burst, is the utilization of a Digital Signal Processor (DSP). The DSP approach is currently used for a particular type of sonobuoy known as the NUAMP. Although the DSP serves well its intended purpose, it possesses drawbacks with regard to the address and command detection because of its cost, and the need of additional electrical and RF power requirements thereby reducing the effectiveness to accurately detect valid sonobuoy addresses. Commonly for the DSP to accurately detect a valid sonar code, the RF power contained in the tone burst from a NUAMP sonobuoy must be greater than xe2x88x9245 dBm, whereas the standard tone detector, sometimes identified as DICASS, responds and accurately processing of tone burst having a power level down to xe2x88x9285 dBm. Because the tone burst usually lasts less then 250 millisecond, the amount of time data is digitized in the DSP approach is limited, and therefore, the frequency bin width is also limited. It is desired to provide for a tone decoder, particularly suited for sonobuoy applications that does not suffer the drawbacks of the Digital Signal Processor (DSP).
It is a primary object of the present invention to provide for a tone decoder that detects for the information, comprised of frequency components, contained in a tone burst even in the presence of interfering frequency and noise, while at the same time does not suffer any system degradation typically created by analog components, as well as detects the information contained in tone burst signals having power levels as low as xe2x88x9285 dBm.
It is a further object of the present invention to provide for a phase lock loop circuit having an oscillator responsive to a control signal generated by a microprocessor containing stored quantities that may represent either a predetermined address contained in the tone burst, the length of the duration of address information, and whether a particular time delay occurs before the command information becomes present in the tone burst signals.
This invention is directed to a frequency detection circuit for measuring information comprised of frequency components contained in tone burst signals even in the presence of interfering frequencies and noise and is particularly suited for sonobuoy applications.
The frequency detection circuit comprises a phase lock loop circuit and a microprocessor. The phase lock loop circuit has an oscillator having a free-running frequency with the phase lock loop circuit also having input, control, and output stages. The input stage receives the tone burst. The control stage is responsive to a control signal to determine the free-running frequency. The output stage provides an output signal having a frequency corresponding to the frequency of the tone burst. The microprocessor has at least one stored quantity representative of a predetermined frequency. The microprocessor has an output stage and an input stage with the input stage thereof connected to the output stage at the phase lock loop circuit. The output stage of the microprocessor is connected to the control stage of the phase lock circuit and provides a control signal thereto. The microprocessor has means for measuring the frequency of the output signal of the phase lock loop circuit and generates an output signal when the stored predetermined frequency equals the frequency of the output signal of the phase lock circuit. The microprocessor also has means for measuring the length of the tone burst. The measured frequency and length of the tone burst can be advantageously outputted by the microprocessor in serial form.